The size of microelectronic and microelectromechanical systems continues to decrease as a result of improved integration and microprocessing techniques. However, the macroscopic power systems currently employed to power these microdevices are much larger than the devices themselves and require complex circuitry. Although the search for micropower sources has recently raised an increasing amount of interest, the demand for suitable small-scale power system that meet microsystem power and energy requirements has yet to be fulfilled. In most applications, power supply miniaturization advanced to the microdevice scale would provide more control over the power delivery to each component of the microsystem and would also simplify electronic circuitry. The incorporation of a micropower source directly into microsystems that also integrate communication and signal-processing components would offer the advantage of complete autonomy, a critical feature in many applications such as microsensors. (J. Long, B. Dunn, D. Holism and H. White, Chem. Rev. 104: 4463 (2004)). One crucial issue related to micro-power sources is to provide enough energy and power to all the components for the remote microsystem to function while minimizing the size of the power system. As constituent materials and fabrication techniques often restrict battery thickness, system optimization usually consists in minimizing footprint occupancy while meeting the energy and power requirements. This challenge opens opportunities for the development of fabrication technologies for materials in the micro and nano scale.
Existing Energy Storage Solutions
a. Thick Film Polymer Batteries
High-energy density primary and secondary batteries of relatively thin dimensions are currently commercially available. (J. L. Souquet and M. Duclot, Solid State Ionics, 148: 375 (2002)). These thick-film batteries are constructed with polymer electrolyte films laminated to the positive and negative electrodes and packaged with polylaminate aluminum/polyethylene heats sealable packaging material. Major polymeric electrolytes (W. H. Meyer, Adv. Mater., 10: 439 (1998), J. Y. Song, Y. Y. Wang and C. C. Wan, J. Power Sources, 77: 183 (1999)) include gel electrolytes formed by polymers swollen by lithium salt solutions and solid polymer electrolytes (SPEs) (I. C. Murata, S. Isuchi and Y. Yoshihisa, Electrochem. Acta 45: 1501 (2000)). The overall thickness of this type of flat batteries, including packaging, lies in the 0.3-3 mm range. The size, packaging and electrochemical performance of such cells make then unsuitable for direct application to small electronic circuitry as would be commonplace in sensors and MEMS.
b. Thin Film Batteries
An alternative to further decrease the overall thickness of flat batteries by an order of magnitude to approximately 10 μm lay in the use of microelectronic fabrication techniques, such as sputtering and vacuum evaporation, to fabricate all solid state thin film batteries. This battery technology is based on thin glassy oxide and sulfide electrolytes. These liquid-free electrolytes suppressed the risk of liquid leakage, a critical issue due to the proximity of the power source to the electronic components. Their low ionic conductivity is counter-balanced by low diffusion lengths as a result of reduced film thickness allowed by the microelectronic fabrication techniques. Furthermore, these fabrication techniques allow the deposition of the battery components directly on the microsystem substrate to achieve small footprint and substrate localization to the operating device.
Eveready Battery Company (S. D. Jones and J. R. Akridge Solid State Ionics, 86-88: 1291 (1996)) and Hydromecanique Et Frottements (HEF), in collaboration with the University of Bordeaux (J. P. Terra, M. Martin, A. Levasseur, G. Meunier and P. Vinatier, Tech. Mg., Genie Blear. D., 3342: 1 (1998)), have manufactured rechargeable all state thin film lithium batteries less than 10 μm thick. While the latter based its battery technology on amorphous titanic or molybdenum oxysulfide cathodes, the former utilized TiS2 cathodes. In both cases, the lithium anodes were obtained by vacuum evaporation while the cathodes and electrolytes were deposited by sputtering. The use of a hydrophobic polymer protective packaging increased the overall thickness of the batteries to about 100 μm.
The most successful thin film battery technology has been demonstrated by Oak Ridge National Laboratory (J. B. Bates, N. J. Dudney, B. Neudecker, A. Ueda and C. D. Evans, Solid State Ionics 135: 33 (2000)). This group has developed rechargeable lithium batteries using RF magnetron sputtering (lithium transition metal oxide cathode and UPON electrolyte) and thermal evaporation (Li anode). These batteries, sealed with a protective hermetic multilayer coating of parylene and titanium, presented the advantage of retaining an overall thickness of less than 15 μm. This battery design was further improved to be compatible with the integrated circuit (IC) assembly solder reflow process performed at 250-260° C. The low melting lithium metal anode (180° C.) was replaced by high melting inorganic anodes in Li-ion batteries and by in-situ lithium platted copper anodes in initially lithium-free batteries.
Although these very thin batteries offer long cycle and shelf life, they are unable to satisfy the area energy requirements for microsystem applications. Sputtering techniques prevent the addition of carbon to enhance the electronic conductivity of the semi-conducting cathode, limiting its thickness and therefore its capacity per area. Sequential sputtering of complete electrochemical cells to build on thickness does not afford a solution, as multiple current collectors must be utilized thereby limiting columetric energy density. In addition, sputtering and vacuum evaporation fabrication methods are costly and time consuming due to low film deposition rates in the order of nm to μm/h. Therefore there is a critical need to establish technology with the small footprint of thin film batteries but with thicker electrodes in the range of 25-100 microns to deliver the required energy.
Three Dimensional Batteries
As discussed, existing thick film technology and thin film battery technology offer poor solutions to the majority of micropower applications. Having identified this problem, a number of researchers have instituted studies related to the development of three dimensional battery microstructures. The advantage of such microstructures is that they consume small amount of surface area on the electronic component and allow the development of energy by building the energy storage device in the z or third direction perpendicular from the substrate. However, due to the intrinsic complexity of the lithium battery technology, it is very difficult to assemble such batteries in a reliable manner that enables such structures to be incorporated. To date, no one has identified a means to do so and demonstrated a working cell. The lithium battery technology consists of a negative electrode (Li metal), an electrolyte/separator (solid state lithium ion conductor) and positive electrode material. Successively deposited or self assembled architectures are very difficult to achieve in 3 dimensions and have poor prospects for robustness once assembled. The latter point is due to the tendency to form electronic shorts through the electrolyte/separator.
The common theme of all the above techniques is the use of traditional lithium-ion or lithium metal related battery configurations to address a very complex and unique problem. It is readily apparent that a new approach is needed, and that is the subject of this invention.